Many microcontrollers, including the LPC1768, include an ADC and multiplexer on chip. If this is done quickly enough, it’s as if all inputs are being converted at the same time. The user can then select any one of several inputs to the ADC. Instead, the usual practice is to put an analog multiplexer in front of the ADC. We could just use more ADCs, but this is costly and takes up semiconductor space. Once we start working with an ADC, we usually find that we want to work with more than one signal. All rights reserved.ĪDC Converts its Analog Input to Digital Outputįigure 5.1 An example data acquisition system. Fast and Effective Embedded Systems Design. The clock frequency, subject to its own set of constraints, determines how fast the ADC operates. As with so many digital or digital/analog subsystems, there is also a clock inputda continuously running square wave, which sequences the internal operation of the ADC. In one way or other the ADC compares the input voltage with the voltage reference, and comes up with the output number, based on this comparison. Think of this as a ruler or tape measure. The ADC works with a voltage referencedan accurate and stable voltage source. The conversion takes finite time, maybe some microseconds or more, so the ADC needs to signal when it has finished. It is under computer control the computer can start a conversion. This has an analog input and digital output. To the right of the diagram is the ADC itself. Some features of a general purpose data acquisition system are shown in Fig. The ADC almost always operates within a larger environment, often called a data acquisition system. These examples display very different signal characteristics, and it is not surprising to discover that many types of ADC have been developed, with characteristics optimized for these differing applications. Still others, like video or radar, have a very high frequency content. Others, like sound and other vibrations, are periodic, with a frequency range up to tens of kilohertz. Of these some, like temperature, change very slowly. The list of possible analog input signals is endless, including such diverse sources as audio and video, medical or climatic variables, and a host of industrially generated signals. Effectively it “measures” the input voltage and gives a binary output number proportional to its size. Having performed this analogto-digital conversion, we can then use the microcontroller to process or analyze this information, based on the value of the analog input.ĥ.1.1 The ADC An ADC is an electronic circuit whose digital output is proportional to its analog input. We can convert analog signals into digital representation, with an accuracy determined by the ADC. This is where the analog-to-digital converter comes in. Yet it is essential for the microcontroller to have these signals available in digital form. Analog Input 5.1 Analog-to-Digital Conversion (ADC) The world around the embedded system is largely an analog one, and sensorsdof temperature, sound, acceleration, and so ondmostly have analog outputs.
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